1. Field of the Invention
This invention relates to apparatus for pressure testing blowout preventers and more particularly to improvements in wellhead test plugs of a type comprising a body which is lowerable on a pipe string into the bore of a wellhead connected beneath the blowout preventer so as to seal thereagainst when so lowered so that closure means on the preventer may be extended into sealing engagement about the pipe string to enclose an annular space thereabout into which fluid under pressure may be introduced in order to test various parts of the blowout preventer.
2. Brief Description of Prior Art
Blowout preventers are used during underwater drilling operations and are provided at the wellhead to prevent uncontrolled escape of natural gas and/or crude oil from within the well which would occur otherwise at the stage in a drilling operation at which the pressure in the well exceeds the pressure at the drill bit which is known as the "mud pressure" and which is the pressure of the column of water and other matter that extends upwards from the drill bit to the surface. Also a blowout preventer serves to retain the gas and/or oil within a well once the well has been drilled.
Wellhead test plugs are adapted to be fitted to the end of a pipe string, such as the drill bit support column, in place of the drill bit, (optionally such plugs may be positioned in a pipe string above the drill bit so that the test plug can be used without completely removing the pipe string and drill bit from the well) so that they can be inserted into the bore of the wellhead and removed from that bore by respectively lowering and raising the drill bit support column. Also such wellhead test plugs are adapted to be seated upon an annular shoulder or similar abutment within the bore of the wellhead when they have been inserted in the bore of the wellhead and are positioned to seal that bore upstream of (usually beneath) the blowout preventer, and include a sealing ring which is adapted to engage the bore of the wellhead to effect the seal. Such an annular shoulder or similar abutment forms part of a conventional wellhead and its purpose does not need to be explained for a proper understanding of this invention.
In one prior apparatus a cup-type packer is carried about the body for direct sealing engagement with the bore of the wellhead. However, when the plug is installed in an underwater wellhead the close fit of the packer within the riser pipe above the preventer stack which is necessary in order to permit the packer to seal against the wellhead bore makes it necessary to raise and lower the apparatus at a very slow rate.
In another prior apparatus shown in U.S. Pat. No. 3,177,703 an annular seal assembly is carried by a body for lowering into an annular space between the bore of the wellhead and a casing hanger supported therein, in order to seal with respect to both the bore and the outside of the hanger. A cup-type packer is also carried by the body for sealing against the inside of the casing hanger whereby the body is effectively sealed with respect to the wellhead bore and both the seal assembly and preventer may be pressure tested. Although well fluid may be permitted to bypass the seal assembly and thereby enable the body to be raised and lowered more rapidly, it has been found that if the seal between the outer side of the seal assembly and the wellhead bore fails to hold test fluid pressure, such pressure upon leaking into the annulus between the wellhead bore and the casing string suspended from the hanger, can cause the casing string to collapse.
In another prior apparatus shown in U.S. Pat. No. 3,897,824 a seal is carried about the outer portion of a body which includes a sleeve whereupon the seal is slideable to a portion of the sleeve having an expanded diameter so that the seal is urged into contact with the wellhead bore. The device is relatively complex and includes valves and a plurality of sliding members and shear pins.
As a result of the continued use of blowout preventers, on subsea wells in particular, and the necessity for testing such preventers frequently a continuing search has been directed to the development of a relatively simple yet highly reliable blowout preventer wellhead test plug apparatus.